Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1136 AM EDT Wed May 08 2019 Valid 12Z Sat May 11 2019 - 12Z Wed May 15 2019 15 UTC Update... Overall, changes to continuity were relatively small during this forecast update. A blend of the 00Z ECMWF/CMC and 06Z GFS were used as a forecast starting point during days 3-5 (Sat-Mon). Models handle shortwave energy crossing the Great Lakes during this time period similarly, with differences primarily confined to amplitude. Solutions are also similar with respect to cutoff southern stream energy skirting along the southwest U.S./Mexico border Sat-Mon. Primary differences emerge by later in the forecast period, and weighting of ensemble means (ECENS/GEFS) was increased during this time frame to account for increased uncertainty. ECMWF/CMC continue to show a slower/more amplified shortwave crossing the Northeast Tue, with a resultant deeper surface cyclone closer to the Mid-Atlantic/Northeast coastline than the GFS, which is weaker aloft and more suppressed with the surface system offshore. At this time, preferred to lean a little more toward the more amplified solutions at 500 hPa, with a resultant blended solution for this system very close to the CMC. Models also show significant differences with respect to troughing near/off the West Coast by early next week, with a high degree of run-to-run variability lending credence to a mostly ensemble-based forecast. Solutions were closer with southern stream energy as it moves into the Southern Plains early next week, with similar timing and differences primarily confined to whether a consolidated upper low remains or whether the system begins to more quickly shear apart with the energy disbursing across a wider area. Ryan Previous Discussion (issued at 0700 UTC) ...Overview... Expect mean troughing aloft to settle over eastern North America, consisting of a lead amplifying trough reaching the Plains/Mississippi Valley during the weekend and continuing east/northeast thereafter followed by a more shallow trough whose core should track north of the Great Lakes by the middle of next week. The first trough will spread moisture northeastward over the eastern U.S. while giving eastern Texas and the Lower Mississippi Valley at least a brief reprieve from a heavy rainfall threat that should extend through Sat. Meanwhile the pattern over the West takes on a Rex block appearance during the weekend/early next week as an upper ridge drifts over the northern half of the region and an upper low slowly tracks over/near the Desert Southwest and southern Rockies. The upper low will promote multiple days of precipitation over southern parts of the West with activity eventually extending into the southern Plains and vicinity as the feature approaches and reaches the region. Farther westward the models and ensemble means continue to show a trough amplifying off the West Coast during Mon-Wed. ...Guidance Evaluation/Predictability Assessment... For the full forecast domain a blend of mostly operational guidance early followed by a trend to 60 percent 18Z GEFS/12Z ECMWF means by day 7 Wed provided the best starting point to reflect the most agreeable aspects of the forecast while accounting for other cases where spread still exits. This approach yielded a solution that maintained or only modestly refined continuity. For the evolution over the eastern U.S. from Sat onward, guidance seems to be making only a gradual attempt to converge toward an intermediate solution for the system approaching/reaching the Great Lakes during the weekend. There are still meaningful medium-smaller scale detail differences aloft that tend to have fairly low predictability several days out in time. The GFS is still on the stronger side of the spread but aside from the 00Z run (which has an odd evolution aloft Sun-Mon) it has shown some trends toward weakening over the Lakes with greater emphasis on low pressure to the east. Regarding the overall eastern trough aloft and the leading low pressure that eventually tracks off the northeastern coast, the ECMWF/CMC have been the most amplified aloft with surface low pressure generally on the northwestern side of the full spread (similarly for the extent of associated moisture). Thus far an intermediate solution appears best as trends have not yet displayed a pronounced shift to either side of the envelope. Farther upstream guidance has become better defined over the past couple days for a front that should reach the northern Plains by day 5 Mon and continue south/east thereafter. For the time frame involved there are typical differences in southward extent. Consensus has held up fairly well for the upper low tracking along the western U.S.-Mexico border and then in weaker form over the southern Rockies/Plains. Multi-day history has generally favored solutions in the middle to slower half of the spectrum. For the trough amplifying off the West Coast the primary consideration was exclusion of the CMC whose latest runs are questionably fast to bring height falls into the Northwest. By day 7 Wed some GFS runs become somewhat faster than consensus with at least a portion of the trough as well. Beyond that, individual models/ensemble members still suggest considerable uncertainty over how energy will be distributed within the trough--favoring an ensemble mean/blend approach. ...Weather Highlights/Threats... The combination of waviness along a stalled front over the western Gulf Coast/Lower Mississippi Valley, low level Gulf inflow, and height falls aloft will extend the heavy rainfall threat over the region into Sat. Then northeastward progression of low pressure should bring drier weather for a time. Confidence remains lower than desired for the axis of heaviest rainfall over the East from Sat night onward--as far northwest as the Ohio Valley/Northeast or as far south as the Tennessee Valley/Mid-Atlantic. Depending on exact evolution at the surface and aloft over/off the Northeast, highest terrain of New England could see a little snow from this system. Farther west, expect a weakening system to bring mostly light amounts from the northern Plains into Great Lakes during the weekend. The next front dropping southeast from the northern Plains may be accompanied by some rainfall by next Tue-Wed. The upper low affecting the southern parts of the West will bring areas of terrain-enhanced precipitation (mostly rain) from the weekend into early next week. Expect rainfall to increase over the southern Plains Mon-Wed as the feature approaches and reaches the area. However there is currently poor agreement regarding the coverage/intensity of potential showers and thunderstorms. Other portions of the West may see scattered rainfall at times, and then cloudiness/moisture should increase along the West Coast states toward Tue-Wed as the upper trough approaches. Low confidence details of the trough will determine the coverage/intensity of any associated precipitation. Very warm temperatures will continue over the Northwest through the weekend with broad coverage of plus 10-20F anomalies (possibly locally higher on Sat). There may be better potential for record highs/warm lows in the short range period through Fri but some potential may extend into the weekend. The approaching upper trough will lead to a gradual moderating trend next week. On the other hand the system affecting the southern parts of the West will keep the Southwest into southern Plains below normal for highs with some minus 10-20F anomalies. Eastern U.S. temperatures will vary with system progression. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml